The present invention relates to a camera, and in particular to a camera with a distance measuring apparatus having various functions which are selectively utilized to measure information on the distance to an object.
Conventionally, the following two systems are adopted for an auto focus (AF) distance measuring apparatus for a camera, namely, an "active type AF system" using a reference signal light applied onto an object from the camera and a "passive type AF system" using the luminance distribution information on the object.
The former "active type AF system" is capable of measuring a distance to an object in a dark environment or a distance to a low contrast object without a luminance distribution. However, the accuracy for measuring an object at a long distance which cannot be reached by reflection light is low.
The latter passive type AF system, by contrast, can provide stable distance measurement for both an object at a short distance and an object at a long distance. Conversely, however, the passive type AF system is basically incapable of measuring a distance to an object in a dark environment or a distance to a low contrast object.
If both of the "active type AF system" and the "passive type AF system" are adopted, it is possible to accurately measure a distance to an object using one or the other of the active and passive type AF systems.
Nevertheless, in the case of adopting the both systems, the camera is disadvantageous in terms of cost and space. Due to this, there has been little chances of actually producing a camera provided with the both systems.
Under these circumstances, there is disclosed, for example, a technique related to "a light projection type range finder and a photoelectric transfer apparatus" capable of efficiently providing these two systems in a small space. See Jpn. Pat. Appln. KOKAI Publication No. 60-105270.
The photoelectric transfer apparatus disclosed in the above mentioned Japanese patent application is provided with sensor arrays for transferring light to electric charges and two series of capacitors for storing electric charges, and extracts signal light in an active AF manner in accordance with the difference in storage charges between the time at which distance measurement light is projected and that no distance measurement light is projected.
Further, this photoelectric transfer apparatus detects images in a passive AF mode based on the arrangement of the charges stored in the respective sensor arrays.
In U.S. Pat. No. 5,652,926, there is also disclosed a technique related to "a distance measuring apparatus" provided with an active system measuring unit and a passive system distance measuring unit, wherein it is determined whether environmental luminance is at a high level or at a low level, and whereby the active system distance measuring unit is utilized if the environmental luminance is low and the passive system distance measuring unit is utilized if it is high.
This U.S. Pat. No. 5,652,926 also discloses a technique related to "a distance measuring apparatus for a camera" provided with an active system distance measuring unit and an active type distance measuring unit, wherein the active system unit is caused to conduct distance measurement once, if the measurement value of the active system unit is lower than a predetermined value, and whereby the measurement value is adopted if it is higher than the predetermined value, adopting a measurement value of the passive system distance measuring unit.
Moreover, in U.S. Pat. No. 5,915,233, there is disclosed a technique related to a "distance measuring apparatus" wherein a passive system is normally adopted and the quantity of light of reflection signal light is detected using only distance measurement light, so that a distance to an object can be calculated is the object has either low luminance or a low contrast.
Next, the switching between the active system and the passive system according to the conventional technique as well as the problem thereof will be described with reference to FIG. 13.
FIG. 13 is a flow chart showing a normal control sequence for focusing a camera having AF functions of two systems, i.e., the active system and the passive system.
Specifically, until it is detected that a release switch is depressed (in step S1), a sub-routine for displaying the number of modes provided in a finder or at a dram of the camera and that of photographic frames is repeated (in step S2).
Then, if it is detected that the release switch is depressed, light is measured for exposure (in step S3) and an AF sequence is executed (in steps S4 to S10). Based on these steps, focus adjustment (in step S11) and shutter control (exposure control) (in step S12) are conducted.
In this sequence, the active system AF is first conducted in step S4 in order to take full advantage of the active and passive systems. Then, a distance to the object is calculated from a reflection light positional signal obtained by the step S4 (in step S5). Based on the result, it is determined whether to conduct the passive system AF (in step S6).
Thereafter, if the result of the step S6 is "NO", that is, the result of the active AF system is appropriate, focusing adjustment is instantly conducted.
In addition, if the result of the step S6 is "YES", that is, the result of the active AF system is inappropriate, the processing moves to the integrating operation of the passive AF system (in step S7) and focusing control is conducted by calculating a distance using an image signal in steps S8 to S11).
In this sequence, if the result of the step S6 is "YES", the camera executes both the active and passive AF systems. Clearly, therefore, this technique requires quite a lot of time.
As can be seen from the above, a conventional camera having two AF systems of the active and passive systems has disadvantages in that it is necessary to determine which of the measurement results of the active and passive systems should be utilized for focusing. As a result so-called release time lag occurs during the determination, whereby a photographer misses shutter timing due to the time lag.